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Description

Nixie Tap is a minimal device which will elegantly display information in living rooms, work desks and bedrooms.
The goal is to design a completely open product, and manufacture a small batch via Crowdsupply.

Project Logs

Manufacturability

If you wish to bring the product to market, manufacturability is as important as design itself. It's no good if you have a perfect product that can't be manufactured, or a crappy one that's easy to manufacture.

Having this in mind, my goal for 1.2 version was to eliminate as much through hole parts as possible. Why? Most probably someone will have to solder all through hole parts by hand.

Unfortunately, since no one thought of making SMD Nixie Tubes, some things will have to remain through-hole, and will be soldered manually.

Touch sensor - TTP223

I spent quite a lot of time searching for a solution for touch detection.

First it was the piezo sensor, which was unreliable as a tap detection mechanism (although it meticulously detected beats from music), and then a friend suggested TTP223.

I bought 5 pcs of the red board with a big, inviting, touch circle, for 0.1$ each.

ESP-12F module

It works! However...

I learned the hard way that not all modules are equal, and that there is a certain flash mode that you must set (dio, qio), in order that your ESP8266 boots after flashing, and that you don't have a ruined day.

Also, when integrating ESP-12 modules, you should follow Espressif guidelines on PCB design, namely, you should not have copper pour below the module antenna. I left the pour by accident, and now the WiFi signal strength is weaker than it should be.

Future steps

To call it a day, there is only one major change required.

Currently, the device is powered by a 5 V to 170 V DC-DC converter, sourced from eBay. There are no schematics available, so, if anyone wants to reproduce this project, (s)he would have to buy the module directly from eBay.

Furthermore, the dude making the modules can get bored and stop making them. It's not wise to rely on an volatile sources for any kind of volume production.

Piezoelectric
transducers are curious little devices. You can use them both ways, as a
vibration source and as a vibration sensor.

In this project,
piezo crystal is used as a sort of a "knock" sensor, or a microphone.
It’s glued to device walls from the inside.

ESP8266 is
continually polling (yeah, not too optimized) the A0 input. When device is
tapped with a finger, piezo picks up the vibration, and generates a voltage
spike. ESP8266 samples the voltage, and if it's over detection threshold, it
considers that a tap event has occurred.

This project is
designed around IN-12B nixies (same as IN-12A but with a decimal dot in front
of the number). Its compact size
makes this tube perfect for a desktop display application.

To make a Nixie, you
take conductors shaped like numbers and place them in a Neon - filled glass
tube. When the voltage
across conductors gets over a certain threshold (striking voltage), gas around
the conductor will light up, due to corona effect.

In principle there
are two ways to drive a Nixie:

direct drive

multiplexing

In practice,
multiplexing uses fewer pins, but may suffer from audible noise, because the nixie
is a mechanical beast which can behave like a speaker due to vibrations.

Since I don't want my clock to buzz at me, I opted for a direct drive strategy.

To sustain the digit
glow, a sufficient current needs to flow through the tube. Based on the tube datasheet and
experimentation, the circuit is currently tuned to the following operating
point:

NOS single piece sockets have a huge advantage when it comes to PCB assembly. These sockets are held together by some sort of resin, which makes soldering easier. However, they are relatively bulky, and I wanted to stay away from NOS as much as possible.

Generic pin sockets are readily available, cheap, but probably require hand soldering, so this option may end up being more expensive than NOS option.
In any case I chose this option:

Brand name sockets are also an option, but I gave up on these, as they are more expensive than generics.

ESP8266 is low on
available GPIO, and since the project needs 11 x 4 = 44 signals, really the
only way was to use some sort of a port expander or SPI driver.

Microchip's HV5812
is a nice IC, SPI input, 20 parallel outputs:

When LOW, HVOUT goes
to GND, Nixie sees more than 130 V and turns on. When HIGH, HVOUT
goes to ~VPP (in our case 80V). Nixie now has 130 V - 70 V = 60 V on it, which
is not sufficient to turn it on.

Now we just connect
HV5812 to ESP8266 SPI and all is fine and dandy! But…oh…not so fast:

Fortunately, there
is a component which operates on 3.3V SPI and can output 5V SPI: the jellybean
74HC595!

So, we buy two
HV5812 (40 digits) and one HC595 (4 dots), and we get level shifting for free!

Only thing is, HC595
needs external high voltage transistors, such as MMBTA42.

An obvious question
pops up: why didn't I use 6 x 74HC595? Why do I need HV5812 ICs when HC595s are
available everywhere?

Let's look how much
these parts cost:

Price @1 qty

Price @ 100 qty

Price @ 1000 qty

HV5812

2.32

1.75

1.75

74HC595D,118

0.43

0.2

0.10526

MMBTA42

0.17

0.085

0.0357

[units: $, source: Digi-Key]

HV5812 is 5 times
more expensive than HC595, and the situation only gets worse at higher
quantities. Also, for some reason, Microchip price breaks end at 100 pcs. Maybe they don't like selling high
quantities through Digi-Key?

Another funky thing
about economies of scale: 1000 pcs of HC595 cost the same as 526 pcs! Go
figure.

Let's now compare
total cost for two driver solutions (pick and place costs excluded):

2 x HV5812 + 1 x HC595 + 4 x MMBTA42

6 x HC595 + 44 x MMBTA42

Price @1 qty

Price @ 100 qty

Price @ 1000 qty

Solution 1

5.75

4.04

3.75

Solution 2

10.1

4.94

2.2

[units: $, source: Digi-Key]

Bulk of the cost for Solution 2 is in the transistors, which make up around 75% of the cost for all price breaks.

Device design will be as simple as possible, so that it can fit any environment: work desk, coffee shop, or library. This means no backlight LEDs, no huge logo, only streamlined material shapes.

So, we need:

- "raw" materials

- streamlined design

- manufacturable design

This log is work in progress.

Prototype 1

The starting idea was to use aluminium and wood, and after short brainstorming this came out:

Natural materials are present, but enclosure is not smooth, and manufacturing is complicated, due to two materials. At the same time, top aluminium plate looks like it's missing something, a CNC marking, which further complicates the manufacturing.

Prototype 1.1

Frustrated by the previous combination of materials, I moved on to a single wood (or Aluminium) block:

Now everything is smooth, manufacturing is simpler. Enclosure can be painted glossy black, or manufactured in a nice wood, such as mahagony, which would give it that nice "Rolls-Royce dashboard" look.

Before plunging in to CNC wood milling, I decided to manufacture this prototype on a 3d printer in a local service in Serbia. After a short wait, it arrived: This prototype illustrated why you should always prototype before production.

It appears that Nixie cutouts are too big, which means that the whole device can be scaled down a little. Also what became apparent after playing around is that the device itself looks way nicer when tubes are recessed:

On the rear side, I planned to have a cover plate, made from FR4 or Aluminium. This cover plate is attached to the enclosure via two screws, one for left and one for right side:

The HV5812 is rated to 80V, yet your HV_MID is going to be 85V if the HV is 170V. Although the absolute maximum rating is 90V, maximum is 80V, you're risking premature failure. Are you considering limiting the output of the HV module to 160V?